Evaluation of SSI and WSI Variables


Storm Severity Index and Winter Severity Index are important measures in comparing individual storms and annual winter weather severity. Currently, there is no standard set of variables used in connection with these indexes. While an agency can review and compare its variables with similar variables used by other agencies, the lack of a uniform set of variables measured in a consistent manner precludes agencies from effectively comparing the winter-related inputs such as the labor and materials used to perform winter maintenance.


  • Identify state, national and international agencies using SSI and/or WSI.
  • Determine the variables and measurement methods used to create the indexes. (A variable of particular interest to Clear Roads states was freezing rain.)
  • Gather statistical data resulting from analysis of each variable for consistency and variability.
  • Recommend the most reliable variables, with the highest correlation to storm severity, for developing SSI/WSI.
  • Create a flexible spreadsheet tool that allows agencies to develop a state-specific SSI/WSI.


Researchers created a step-by-step guide and a flowchart tool to help agencies identify or develop severity index methods that fit their needs and their available data sources. Once implemented, these severity indexes will allow winter maintenance managers, researchers and transportation agencies to more effectively compare winter operations among localized areas, districts and states. Agencies are also better able to compare individual storms across years, which in turn allows them to more accurately evaluate the impacts of innovations and new winter maintenance methods.

The Final Webinar took place on December 22, 2020.